1. Field of the Invention
This invention relates to a cascade control apparatus for controlling furnace temperature by using adjustment input. In particular, the invention relates to a cascade control apparatus of learning type for controlling the temperature of a furnace operated in repetition of a preset pattern of temperature change with time, the apparatus using adjustment input which is renewed by its values in the preceding cycle of operation and the result of a preceding cycle of operation as compared with preset pattern.
2. Description of the Prior Art
There are furnaces which are operated in repetition of a preset pattern of temperature change with time. As a means for improving the temperature change of such repetitively operated furnace at each cycle of repetition by learning the temperature change in the preceding cycle so as to bring the actual temperature change close to the preset pattern, the inventors disclosed "a furnace temperature control apparatus using adjustment input" in their U.S. Pat. No. 5,099,442 issued Mar. 24, 1992.
The apparatus of U.S. Pat. No. 5,099,442 issued Mar. 24, 1992 will be briefly reviewed by referring to FIG. 2. A furnace 1 is heated by heaters 2, and temperature sensors 3 measure the heater temperatures and inside temperatures of the furnace 1. The output from the temperature sensors 3 are applied to a controller (corresponding to a primary controller 12 of the invention) through a converter 4 as state variables (x). Of the temperature sensor outputs, values of furnace inside temperature (y) are stored in an output memory 6. The preset pattern of furnace inside temperature change with time is stored in a preset temperature value memory 7 in the form of preset values (r) of the inside temperature.
In response to input signals including the preset values (r) from the preset temperature value memory 7, adjustment input (r) from an adjustment input memory 11, the above-mentioned state variables (x), and the furnace inside temperatures (y), the controller produces output or manipulating signals (.tau.) to be applied to the heaters 2. In the case of the previous control apparatus of U.S. Pat. No. 5,099,442 issued Mar. 24, 1992, secondary controller 35 of FIG. 2 is not used, and the manipulating signals (.tau.) are distributed to a plurality of heaters 2 through a D/A converter 5. Here, the adjustment input (r) is determined in such a manner that the error (e) between the preset value (r) and the furnace inside temperature (y) is reduced as the cycle of operation of the furnace 1 is repeated. The symbol (.tau.) for the manipulating signal corresponds to the symbol (u) in the above U.S. Patent.
Each time one cycle of the repetition, namely an entire pattern of furnace inside temperature change with time, such as the entire pattern of Curve 3 of FIG. 6(C) over 0-25 min, is finished, an adder 8 finds errors (e) between the preset temperature values (r) and the corresponding furnace inside temperatures (y) in the cycle Just finished. The errors (e) are applied to a dual system model 9 with respect to an enlarged system 40 (FIG. 1) consisting of the furnace 1 and the controller 12. Based on the outputs (v) from the dual system model 9 in response to the application of the errors (e) thereto and the values of the adjustment input (r) in the preceding cycle, i.e., the cycle just finished, an adjustment input generator 10 renews the values of the adjustment input (r) and the contents of the adjustment input memory 11 are replaced with the thus renewed values. Then, the next cycle of repetition starts.
The furnace temperature control apparatus using the adjustment input performs a kind of learning type control, because at the end of each cycle of the repetition it renews the values of the adjustment input for one cycle and then proceeds to the next cycle of repetition. Thus, such control apparatus has an advantage over non-learning type in that the actual furnace inside temperature change pattern is brought closer to the preset pattern each time the cycle of the repetition takes place.